Enhancing superconductivity in A3C60 fullerides

TL;DR

This paper explores how light-induced structural changes in A3C60 fullerides can enhance superconductivity by creating an interaction imbalance that stabilizes intra-molecular singlet pairs, pushing the material into a stronger coupling regime.

Contribution

It introduces a theoretical mechanism where interaction imbalance enhances superconductivity in A3C60, supported by first-principles calculations of electronic structure changes.

Findings

Interaction imbalance enhances superconductivity.

Deformation induces a favorable interaction imbalance.

Superconductivity is pushed into a strong-coupling regime.

Abstract

Motivated by the recent experimental report of a possible light-induced superconductivity in A3C60 at high temperature [Mitrano et al., Nature 530, 451 (2016)], we investigate theoretical mechanisms for enhanced superconductivity in A3C60 fullerenes. We find that an `interaction imbalance' corresponding to a smaller value of the Coulomb matrix element for two of the molecular orbitals in comparison to the third one, efficiently enhances superconductivity. Furthermore, we perform first-principle calculations of the changes in the electronic structure and in the screened Coulomb matrix elements of A3C60, brought in by the deformation associated with the pumped T1u intra-molecular mode. We find that an interaction imbalance is indeed induced, with a favorable sign and magnitude for superconductivity enhancement. The physical mechanism responsible for this enhancement consists in a stabilisation of the intra-molecular states containing a singlet pair, while preserving the orbital fluctuations allowing for a coherent inter-orbital delocalization of the pair. Other perturbations have also been considered and found to be detrimental to superconductivity. The light-induced deformation and ensuing interaction imbalance is shown to bring superconductivity further into the strong-coupling regime.